This invention generally relates to a method and apparatus for connecting two single-phase uninterruptible power supplies to provide two-phase electrical power.
The electric power utility typically provides three distinctly different electric services to their customers, depending on their needs. The services differ in the number of power phases. A single phase service is utilized mostly in the power range below 2 kVA while a three phase service is utilized in the power range above 10-15 kVA. In the power range between 2 and 10-15 kVA, two phase service is the most economical and is widely utilized for electronic equipment and in residential and light commercial power distribution.
Single and three-phase uninterruptible power supplies (UPSs) are known in the art and can readily be used to power equipment operated from single phase or three phase power lines. The phase relationship between the power phases in a three-phase UPS is fixed at 120 degrees and 0 degrees in a single phase UPS.
In a two phase distribution system, the phase difference between both phases is not known a priori. It can be either 120 degrees, 180 degrees, or 240 degrees. The phase difference, whatever it is, must be maintained. Because of this uncertainty with respect to phase difference, two phase UPSs which maintain the same phase difference at their output as the electric utility did not exist prior to the present invention.
The prior art solution for two phase power is to use power between phases with a single phase UPS and to generate on the output several different voltages, all in phase with the input. The disadvantage of this approach is costly load rewiring, oversizing due to the increased currents in one of the output conductors, and expensive maintenance bypass installations.
Single phase UPS systems for minicomputers are generally operated from a 208 V input power source. Their output is either 208 and 120 (208/120) or 240/120 volts. The output of these UPSs provides two different voltage values (e.g., 208 and 120 volts), but the voltages are in phase with each other; in other words, the voltage peaks occur at the same instant, making the UPSs single phase.
In single phase 208 volt output UPSs, a new neutral at the secondary winding of the output transformer must be created so that both 208 volt and 120 volt power may be derived. 208 volt is developed from one line to neutral, and 120 volt from the other line to neutral. This configuration results in the neutral conductor carrying extra current, requiring that it and the components in the UPS be oversized. This oversizing is necessary in order for the single phase unit to operate properly and safely, but represents an expense that need not be incurred with use of a two phase UPS.
A second problem associated with protecting minicomputers with a single phase UPS concerns the periodic need for the computer to bypass the UPS and to draw power directly from the utility service. A mechanical device called a maintenance bypass switch, which is external to the UPS, makes it possible for the computer load to bypass the UPS completely and draw current directly from the power line without any interruption of power in the process. This is down so that the UPS can be disconnected for servicing without interrupting computer operations.
Accomplishing this bypass, without risk to the computer and its data, requires that the output from the electrical service be of identical voltage and phase at the instant of bypass to that being drawn from the UPS. In a single phase UPS, achieving this match requires the addition of an isolation transformer to the electrical service outlet that is feeding the maintenance bypass. The transformer must have a secondary winding absolutely identical to the output winding of the UPS.
Installing such a transformer is an expensive process. The transformer must be oversized by at least two times in order to accommodate the way computers draw power. A separate electrical service for the transformer with circuit breakers and a separate ground are also required to meet building safety codes.
Another drawback of using a single phase UPS is that the power distribution to the loads shall be changed from a two-phase type to a single phase type.
For all those reasons, a significant cost advantage can be achieved with a two-phase UPS.
Single phase UPSs are well known in the prior art and include UPS Model 286/LAN and Model 15A made by VITEQ Corp., the ON-GUARD series of UPSs made by CLARY Corp., the 1200 series of UPSs made by Toshiba, and many similar products. The basic block diagram of this type UPS is shown in FIG. 1. It consists of an input rectifier/AC-DC converter 1, DC storage capacitor 2, battery 3, battery charge/discharge circuit 4, DC-AC inverter 5, and bypass switch 6, all performing power conversion under logic control. Charge/discharge circuit 4 allows the battery to discharge when the converter can not maintain sufficient voltage on capacitor 2 and to charge the battery when this voltage is within the regulation range.
As shown in FIG. 1, there are four distinct logic functional blocks. One block is converter control logic block 7 which takes an input AC signal and an output DC signal and produces a train of pulses to converter semiconductor switches within AC-DC converter 1 in such a way that the DC voltage on capacitor 2 is maintained within a narrow regular range. The second block is inverter control logic 8 which takes three signals on its input to generate a train of pulses to semiconductor switches within DC-AC converter 5 to produce a sine waveform voltage on the DC-AC inverter output. The input signals to inverter control logic 8 are a DC voltage to inputs A and B, an AC feedback voltage from the output of the DC-AC inverter to inputs C and D to maintain the output voltage in the desired narrow range, and frequency and phase reference signals to inputs E and F. An inverter enable signal also is supplied to input G. These signals force the inverter to produce a voltage waveform on its output which has the same frequency and phase as the input voltage to the UPS and is necessary for the proper operation of bypass switch 6 which transfers the load between the input power line and inverter in case of an overload, UPS failure, or during maintenance. All such transfers must be in phase and synchronous.
The frequency and phase reference signals are produced by phase-locked loop circuit 9 (PLL) which can supply to the inverter control logic several signals identifying frequency and phase. PLL 9 takes the voltage on the UPS input, conditions it to a square waveform logic voltage level and then generates the same frequency and phase difference as the input voltage. When input power fails, PLL 9 continues to provide the frequency reference which is close to the input voltage frequency under normal conditions. When input power is restored, PLL 9 resynchronizes the reference signals to the inverter so that the inverter output frequency is again synchronized to the input frequency with no phase shift. The speed of resynchronization is controlled so that frequency sensitive electronic loads will not be out of synch during resynchronization process.
Logic blocks 7, 8, and 9 have digital converter status, inverter status and PLL status signals such as input voltage out of range, not phase locked, or output overload which are supplied to enable logic block 10 which may be a digital decoder hardwired or microprocessor based, which enables converter or inverter operations.